If healthy young people stop eating for a week, their ability to perform intense exercise—like running fast or cycling hard—gets worse by about 13%, even though their muscles are still just as strong.

What we've found so far is that a 7-day fast may reduce aerobic endurance in healthy young adults. The evidence we've reviewed leans toward a decline in performance during intense aerobic activities, like running or cycling, even if muscle strength remains unchanged [1]. Our analysis of the available research shows that when healthy young adults stop eating for a week, their ability to sustain high-intensity aerobic exercise drops by about 13% [1]. This suggests that fasting may affect how well the body uses energy during endurance efforts. Importantly, this reduction in endurance happens even though muscle strength does not appear to decline—meaning the muscles themselves are still capable, but the body’s overall capacity to perform prolonged, intense work is lower. We don’t yet know exactly why this happens, but the evidence we’ve reviewed points to a change in energy metabolism during fasting. Without incoming calories, the body shifts to using stored fat and ketones for fuel, which may not support high-intensity aerobic output as effectively as carbohydrates. However, since we only have one assertion to draw from—backed by 38 supporting reports and none that refute—it’s important to recognize that our current view is limited. Based on what we’ve reviewed so far, fasting for seven days seems to impact aerobic endurance, but we can’t say how much this affects real-world performance or whether adaptations occur over time. There’s not enough evidence to determine if this effect is consistent across different people or types of exercise. Practical takeaway: If you're planning intense workouts or endurance training, going without food for a week might make those efforts feel harder, even if you’re physically strong.